Mass spectrometers having an elongated conductor set, typically quadrupole mass spectrometers (which have four rods) have been in common use for many years. It has become common to use such rod sets in tandem in a vacuum chamber. In many such instruments there are four rod sets, referred to as Q0, Q1, Q2 and Q3. Rod set Q0 receives ions and gas from an ion source and has a radio frequency voltage (RF) only applied to it, to act as an ion transmission device while permitting gas therein to be pumped away. Rod set Q1 has RF and DC applied thereto, to act as a mass filter, e.g. to transmit a desired parent ion. Rod set Q2 has collision gas supplied thereto, to act as a collision cell for fragmentation of the parent ions, and typically has only RF applied thereto. Rod set Q3 has RF and DC applied thereto to act as a scannable mass filter for the daughter ions produced in collision cell Q2.
In tandem mass spectrometers of the kind referred to above, and also in other mass spectrometers, gas within the volumes defined by the RF rod sets Q0 and Q2 improves the sensitivity and mass resolution by a process known as collisional focusing, described e.g in U.S. Pat. No. 4,963,736. In that process, collisions between the gas and the ions cause the velocities of the ions to be reduced, causing the ions to become focused near the axis. However the slowing of the ions also creates delays in ion transmission through the rod sets, and from one rod set to another, causing difficulties.
For example, when rod set Q0 transmits ions from an atmospheric pressure ion source into rod set Q1, the gas pressure in Q0 can be relatively high (e.g. above 5 millitorr for collisional focusing), and collisions with the gas can slow the ions virtually to a stop. Therefore there is a delay between ions entering Q0 and the ions reaching Q1. This delay can cause problems in multiple ion monitoring, where several ion intensities are monitored in sequence, at a frequency which is faster than the ion transit time through Q0. In that case the signal from ions entering Q1 may never reach a steady state, so the measured ion intensity may be too low and may be a function of the measurement time.
Similarly, after daughter ions have been formed in collision cell Q2, the ions drain slowly out of Q2 because of their very low velocity after many collisions in Q2. The ion clear out time (typically several tens of milliseconds) can cause spurious readings (e.g. interference between adjacent channels when monitoring several ion pairs, i.e. parent/fragments, in rapid succession). To avoid this, a fairly substantial pause time is needed between measurements, reducing the productivity of the instrument. The extended ion clear out time can also cause spurious peak broadening.